In such systems, the data to be transmitted are converted, either singly or in groups, into successive symbols of duration .DELTA. known as the Baud interval to constitute a base band signal which is then used to frequency or phase modulate a carrier of constant envelope amplitude. Most systems of this kind are known as frequency shift keying (FSK) or phase shift keying (PSK) systems which use symbols having a simple rectangular shape.
Because of the phase or frequency modulation, such systems have the drawback of using a transmission signal whose pass band is considerably larger than the pass band occupied by symbols which use single or residual sideband modulation. The occupied band width decreases with increasing modulation index, but its minimum remains close to the width occupied by double sideband amplitude modulation.
One known way of reducing the bandwidth occupied by a frequency or phase modulation transmission system is to reduce the bandwidth of the signal in the base band. For example, minimum shift keying (MSK) transmission systems are known in which the symbols used are triangular in shape in order to give the base band signal a sawthooth appearance. Another example is constituted by sinusoidal frequency shift keying (SFSK) transmission systems in which the symbols are sinusoidal in shape, giving the base band signal the appearance of a series of sinusoidal arches. In both cases each symbol corresponds to an increase or a decrease of carrier phase by .pi./2, and this limits the number of possible states for each symbol to two, thereby preventing any increase in transmission speed by adopting a greater number of discrete values for the transmitted symbols.
It is also known to apply the conventional data transmission arrangements to the base band signal to reduce the width of the occupied band, i.e. to employ partial response coding and filtering in accordance with Nyquist criteria. However, that results in increasing the number of discrete values possible at the receiver end for a single transmitted symbol value.
Preferred embodiments of the present invention provide a data transmission system using a modulated carrier of constant envelope amplitude which occupies small frequency bandwidth relative to its data rate, and which avoids the limitations of the systems mentioned above.
In such preferred embodiments, the transmitter and receiver portions are easily realised in digital form using large scale integration (LSI) microcircuits.